A pumper/installer/designer I’ve worked with for years posed a question to me about the design loading rate for sandy clay loam soils. I answered his question at the time, but thought the topic should be shared with a larger audience because I am sure he is not the only person who has wondered about loading rates for what I will term the clay loam soils.

Here’s the situation I was asked about: The contractor’s state code used a design loading rate of 0.5 gallons per day per square foot (gpd/sq ft) for sandy clay loam soils, and he wondered if that was what I would use for a design number. He also indicated for clay soils their design rate is 0.2 gpd/sq ft.

Here is the answer I gave him:

Whenever the clay content in the soil gets above 20 percent, the clay properties begin to take over the overall soil characteristics. So my answer is that the sandy clay loam soil should be sized at .24 gpd/sq ft or less. People get confused because when they feel the soil to determine its texture, they note a lot of grittiness, which is due to the sand. But the clay percentage is high enough that it drives the infiltration characteristics of the soil.

During soil evaluation in Minnesota and Wisconsin, if the sandy clay loam soil has moderate to strong structure and is friable or very friable, a 0.45 gpd/sq ft loading rate is allowed. The friable consistence usually would occur only at the surface and would become firm or very firm as you get deeper in the soil, which would result in a loading rate of 0.24 gpd/sq ft. This is probably a nuance that only an experienced site evaluator or soil scientist would easily identify. So at most I would use 0.2 gpd/sq ft, and if I knew it was highly shrink swell, I may drop it to 0.12 gpd/sq ft.

NEED TO ELABORATE

I thought it would be good to discuss in some detail where these numbers came from and make a few comments on their use. The numbers go back to the original research done at the University of Wisconsin in the late 1960s and early 1970s. They first showed up in research reports and then in a classic paper by my early mentor and colleague, Professor Johan Bouma. As the Small Scale Waste Program at Wisconsin continued to research, the numbers were expanded and refined through the work of a number of people, but they are most often associated with Johan’s successor Jerry Tyler. Most design tables with design loading rates based on soil description follow closely the numbers Jerry provided in a series of research papers in the 1990s.

In Johan’s original paper, he already began separating soils into different categories based on soil texture using a method he devised for testing undisturbed soils in the field. I won’t bore everyone with the details but the method involved carving out a soil column and then using a gypsum-water mixture, applying to the soil surface at the desired depth. The mixture hardened and the rate of water movement was measured through the “crust.” The moisture condition of the soil underneath was compared with values measured beneath operating drainfields. From the experience of doing a number of these tests, I can tell you it is not a trivial operation!

With these results, soils seemed to fall out into several different categories. Since the research studies used the metric system to measure the rates of acceptance, they were characterized in centimeters/day (cm/d) of infiltration. A flow rate of 1 cm/d is equivalent to 0.24 gpd/sq ft. and this rate corresponded with clay, sandy clay and silty clay. For clay loam, sandy clay loam and silty clay loam textures, the rate was about 2 cm/d or about 0.48 gpd/sq ft. But it was also noticed that for some of those soils, the rate was faster and it seemed to be related to structure. This led to additional research and the refinement of the numbers based on soil structure that are in use today. The research also showed a lot of variability, which makes prediction of the flow rate difficult for even the most experienced.

NUMBERS LINE UP

The original 0.24 gpd/sq ft (1 cm/d) rate represented the lowest number measured and was at a soil moisture content very close to saturation. This means that the rate is controlled by the soil and not by application method or how clean the effluent is. So taking reductions in treatment area for soils with increasing clay contents does not make sense, because this is what the soil will accept. Does this mean we should not have as clean an effluent as possible? No; but what it does mean is the area necessary for treatment and acceptance should not be reduced.

From the perspective of my colleague’s question, it appears the state requirements are in line with the accepted numbers; 0.2 gpd/sq ft is rounded down from 0.24 and 0.5 is rounded up slightly from 0.48 gpd/sq ft. How the designer chooses to look at the numbers, though, will have an impact on the longevity and success of the system.

In my answer, I took a conservative approach based on knowledge of soils in the area in question and recognition of the soil variability. I am sure a number of you would point out that use of 0.5 gpd/sq ft would be appropriate to fit the regulatory requirement and you would be correct. It just highlights that design decisions are often not as clear as people assume they are.